As I discuss in this question, by far the most popular school of Hindu philosophy is the Vedanta school, which bases its tenets on the doctrines laid out in the Brahma Sutras, a work by the sage Vyasa that summarizes and systematizes the philosophical teachings of the Upanishads. You can read the Brahma Sutras here. Now in the time of the famous Sri Vaishnava Acharya Ramanujacharya, the most popular commentary on the Brahma Sutras was Adi Shankaracharya's Brahma Sutra Bhashya. But as I discuss here, Ramanujacharya felt that Adi Shankaracharya had twisted the meaning of the Brahma Sutras to suit his Advaita philosophy, so in writting his own commentary on the Brahma Sutras he sought out as many pre-Shankara sources as he could, for instance reading Baudhayana's ancient Vritti on the Brahma Sutras.
Another source Ramanujacharya used was Tanka, aka the Vakyakara. Tanka was a sage who wrote an ancient Vakya or commentary on the Chandogya Upanishad. Now in this section of the Sri Bhashya, Rananujacharya's commentary on the Brahma Sutras, Ramanujacharya quotes Tanka's discussion of the Sadhana Saptaka, or sevenfold discipline of Bhakti Yoga:
The Vâkyakâra also declares that steady remembrance results only from abstention, and so on; his words being 'This (viz. steady remembrance= meditation) is obtained through abstention (viveka), freeness of mind (vimoka), repetition (abhyâsa), works (kriyâ), virtuous conduct (kalyâna), freedom from dejection (anavasâda), absence of exultation (anuddharsha); according to feasibility and scriptural statement.' The Vâkyakâra also gives definitions of all these terms. Abstention (viveka) means keeping the body clean from all food, impure either owing to species (such as the flesh of certain animals), or abode (such as food belonging to a Kândâla or the like), or accidental cause (such as food into which a hair or the like has fallen). The scriptural passage authorising this point is Kh. Up. VII, 26, 'The food being pure, the mind becomes pure; the mind being pure, there results steady remembrance.'
Freeness of mind (vimoka) means absence of attachment to desires. The authoritative passage here is 'Let him meditate with a calm mind' (Kh. Up. III, 14, 1). Repetition means continued practice. For this point the Bhâshya-kâra quotes an authoritative text from Smriti, viz.: 'Having constantly been absorbed in the thought of that being' (sadâ tadbhâvabhâvitah; Bha. Gî.VIII, 6). By 'works' (kriyâ) is understood the performance, according to one's ability, of the five great sacrifices. The authoritative passages here are 'This person who performs works is the best of those who know Brahman' (Mu. Up. III, 1, 4); and 'Him Brâhmanas seek to know by recitation of the Veda, by sacrifice, by gifts, by penance, by fasting' (Bri. Up. IV, 4, 22).
By virtuous conduct (kalyânâni) are meant truthfulness, honesty, kindness, liberality, gentleness, absence of covetousness. Confirmatory texts are 'By truth he is to be obtained' (Mu. Up. III, 1, 5) and 'to them belongs that pure Brahman-world' (Pr. Up. I, 16).--That lowness of spirit or want of cheerfulness which results from unfavourable conditions of place or time and the remembrance of causes of sorrow, is denoted by the term 'dejection'; the contrary of this is 'freedom from dejection.' The relevant scriptural passage is 'This Self cannot be obtained by one lacking in strength' (Mu. Up. III, 2, 4). 'Exultation' is that satisfaction of mind which springs from circumstances opposite to those just mentioned; the contrary is 'absence of exultation.' Overgreat satisfaction also stands in the way (of meditation). The scriptural passage for this is 'Calm, subdued,' &c. (Bri. Up. IV, 4, 23).
My question is, what is the context of Tanka's discussion of the Sadhana Saptaka? Ramanujacharya quotes many scriptures to support Tanka's quote, but Tanka's Vakya was a commentary on the Chandogya Upanishad. So what verse was Tanka commenting on that led to him explaining the sevenfold discipline of Bhakti Yoga?
Now Tanka's Vakya on the Chandogya Upanishad is long lost, so we can't be absolutely certain what the context was. But are there any verses in the Chandogya Upanishad which would be particularly relevant to a discussion of the factors of Bhakti Yoga? Have any scholars speculated on what the verse was?